1. Field of the Invention
This invention relates to nuclear fuel pellets including UO.sub.2 and UO.sub.2 -Gd.sub.2 O.sub.3, and more particularly to nuclear fuel pellets having a satisfactory solid-solution state, greater grain diameters and a second phase precipitated in grain boundaries. This invention also relates to a method of manufacturing the above-described nuclear fuel pellets.
2. Description of the Prior Art
As for nuclear fuel to be loaded into a light water reactor or a fast breeder reactor, intactness of fuel has been confirmed at a high burnup level ever experienced in a reactor. However, at present, extension of burnup to still higher levels has been planned. This plan inevitably involves the following disadvantages. Specifically, so-called bubble swelling occurs due to fission gas deposited in grain boundaries, i.e., an apparent volume of pellets increases due to bubbles produced in pellets because of gaseous fission products. Thus, PCI (pellet-cladding interaction), which is a mechanical interaction between pellets and a cladding, increases. Further, an inner pressure of a fuel rod increases because of fission gas release from fuel pellets. These phenomena may cause intactness of fuel to be deteriorated.
To avoid these disadvantages, the following techniques have been attempted. Specifically, a fission gas release fraction (a ratio of the released to the produced of fission gas) is suppressed by increasing diameters of pellets grains. This is based on that a fission gas release from pellets is rate-controlled by diffusion of fission gas in pellet grains. However, when the diameters of pellet grains are increased, a creep rate of the pellets is decreased. This provides an adverse effect on PCI.
To increase a creep rate of pellets there have been disclosed two technique (Japanese Patent Applications No. 1-193691 and No. 2-242195) in which a sintering agent consisting of aluminum oxide and silicon oxide is added to uranium dioxide powder so that a second soft phase can be precititated in the grain boundaries of the pellets. In these techniques, the total amounts of the sintering agents to be added are about 0.1 wt % through about 0.8 wt %, and about 0.05 wt % through about 0.4 wt %, respectively.
In general, it has been known that sinterability of a mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3 is lower than that of pure UO.sub.2. Further, when sintering is performed under a given condition, a sintered density and grain diameters of the mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3 become smaller than those in the case of pure UO.sub.2. Further, when sintering is performed in a flowing dry hydrogen, a large number of micro-cracks occur in pellets. To avoid the above-described disadvantages, in the case of UO.sub.2 having Gd.sub.2 O.sub.3 added thereto, sintering is generally performed in a humid hydrogen atmosphere or in a mixed gas atmosphere of carbon dioxide and carbon monoxide. Further, the sintering is performed at a relatively high temperature (1700.degree. C. or higher). However, assume that sintering is performed in such atmospheres and a sintering agent, which consists of aluminum oxide and silicon oxide in the above-described conventional proportion, is added to the mixed oxide of UO.sub.2 and Gd.sub.2 O.sub.3. In this case, pores are generated in pellets, probably due to evaporation of silicon oxides, so that a density of pellets becomes decreased. As the pellet density becomes lower, a thermal conductivity of the pellets degrades. As a result, a fuel center temperature increases in service, so that both bubble swelling and a fission gas release rate are enhanced. This is disadvantageous to the performance of nuclear fuel pellets. Further, it has also been experimentally confirmed that grain diameters of pellets and a solid-solution state thereof can no longer be improved even when a sintering agent of 500 ppm or more is added.